1. Field of the Invention
The present invention relates to camera flash units with improved center beam and field illumination characteristics. More particularly, the present invention relates to camera flash units suitable for use in photographic devices or other camera devices requiring a substantially uniform illumination of a target surface.
2. Background
A camera flash unit needs to provide a good, relatively uniform illumination of a target surface and to avoid a lamp interference problem. Lamp interference results when the light rays reflected off the flash unit's reflective inner walls pass through the light source or its transparent envelope. This problem is described in the book entitled "The Optical Design of Reflectors" by William B. Elmer, Third Edition, Copyright 1989, by TLA Lightning Consultants, Inc. Salem, Mass., pgs. 178-179.
In theory, a flash unit with a parabolic reflector would provide a collimated light beam to uniformly illuminate a target surface located near the reflector opening, but only if the light source was a point light source located at the reflector's focal point, and only if the reflected light (but no direct light from the light source) reached the target surface. If a lamp of finite size was used as a light source, then the direct light from this lamp would have to be blocked (as shown in FIG. 1A), and a shadow would be cast in the center of a target plane.
Other flash units have been designed to provide a relatively uniform illumination. U.S. Pat. No. 5,160,192 discloses a flash unit (see FIG. 1B) that includes a reflector 3 composed of a combination of two elliptical shells 4. The optical axes of the shells 4 are separated from one another. An elongated cylindrical light source 6 is located between the shells 4 and has a longitudinal axis 7 parallel to the optical axes of the elliptical shells 4. A Fresnel condenser lens 8 is located at the opening of the reflector 3.
In some of the embodiments the back surface 9 of the condenser lens (i.e., the surface facing the light source 6) is piano and the front surface 10 has a convex, cylindrical shape. Because surface 10 has optical power in one plane only (generally designated the "Y-Z" plane), the condenser lens can control target surface illumination in only that plane.
In other embodiments, the back surface 9 of the condenser lens 8 has a cylindrical shape and the front surface 10 is a concentric Fresnel surface. Thus, the surface 10 is circularly symmetric. The surface 10 of the condenser lens condenses the target surface illumination equally in all directions. The use of the surface 9 can improve illumination profile in one direction, for instance, the horizontal or vertical direction, but the corresponding orthogonal direction is unaffected. Because camera flash light sources are typically elongated, the use of spherical and cylindrical power, separately or in combination, is a compromise attempt to control the illumination distribution in independent, or topologically orthogonal, directions. It is desirable to provide independent illumination of the target surface in two directions.